Usually a switching-type power converter owns multiple advantages, such as high efficiency, high power density and small device volume, and therefore it has been widely applied to computer related products, communication related products, automatic control related products, industrial products and so on. However, the switching-type power converter usually adopts an active switcher or a turn-on or turn-off switching strategy, which may inherently result in the existence of the high frequency electric current ripples. For an actual and real application, it further causes several technical issues, such as the electromagnetic interference (EMI), the unstable voltage output, the raise of transient response, the relatively lower converting efficiency and the shortened component life.
In convention, the simplest way to cancel or eliminate continuous high frequency electric current ripples is to increase the switching frequency or to rise up the filter inductance or the capacitance, so as to minimize or alleviate the affection thereof. However, the increasing of the switching frequency will cause relatively lower conversion efficiency, the rise of the filter inductance or the capacitance will cause relatively larger device volume which is helpless to enhance the overall power density for a power converter. On the other hand, for a power converter having pulsating current ripples at an input end or an output end, such as a Flyback converter, a Forward converter, a Buckboost converter, a Buck converter, a Boost converter, a Sepic converter or a Zeta converter, the simplest way to cancel the pulsating current ripples is to electrically couple with an electrolytic capacitor having a relatively larger capacitance in parallel either at the input end or at the output end, but it increases the overall device volume and shortens the component life for a power converter.
In the state of the art, a current ripple cancelling technology is typically categorized into two types including a passive type scheme and an active type scheme. The passive type current ripple cancelling circuit is a filter circuit which mainly consists of passive components, such as inductor and capacitor, and requires none of additional/external control signals. In the prior arts, the schemes focused on using means of coupling inductor and ripple filter are all categorized as the passive type current ripple cancelling technology. A coupling inductor is often used in a non-isolated Cuk converter. The principal operating concept thereof is to first organize a coupling inductor and then to achieve zero current ripples at the input end and the output end by adjusting the inductive value and the coupling coefficients. Nevertheless, the scheme is unsuitable for a basic Boost, Buck and Buckboost converter which just contains a single inductor. As aforementioned, the current ripple cancelling is achieved by adjusting the coupling coefficients which requires a very sophisticated manufacturing technology which technology is hardly available. In addition, the energy leakage resulted from the winding of the coupling inductor also significantly affects the converting efficiency. Another way regarding the current ripple filter is to use a cancelling circuit consists of both coupling inductor and filtering capacitor. Although the scheme is capable of being applied to a basic converter containing a single inductor, the performance thereof is also dependent upon the coupling coefficients as mentioned above.
As compared with the above-mentioned passive current ripple cancelling technologies, an active current ripple cancelling circuit consists of multiple passive components and active power switches, which additionally requires an external control signal to control the circuit. In the prior literatures in the related technical field, the most common active current ripple cancelling circuit is an interleaving control type parallel power converter. Although the switching strategy based on the interleaving control owns multiple advantages, such as effectively reducing current ripples and sharing input or output currents of a converter, it also bears quite a few limitations and shortages. Except requiring sophisticated control loop layout and costing high price, the scheme is inapplicable to cancel the current ripple in a single-phase converter and to cancel the pulsating current ripple. In addition, the current ripple cancelling performance the interleaving control type current ripple cancelling technology can achieve is totally limited to the variation of switching duty cycle. As if the duty cycle is deviated from the designed operating range, the current ripples generated in the converter fail to be well cancelled.
For the above-mentioned disadvantages, there further raises a passive current ripple cancelling technology. The technology requires none of active power switches which does not increase the costs resulted from the arrangement of active power switches and the related driving circuit. In addition, the current ripple cancelling performance the passive current ripple cancelling technology can achieve is not limited to the output voltage, the output voltage specification or the switching duty cycle originally set in the converter, which causes the technology being applicable to cancel the current ripple in a single-phase converter. Nevertheless, such a passive current ripple cancelling technology is particularly dedicated to cancel the continuous current ripple, and is inapplicable to a switching type power converter having a pulsating current ripple input or output.
There is a need to solve the above deficiencies/issues.